U.S. Pat. No. 7,254,216 B2 discloses a filter assembly for a computed tomography apparatus. The filter assembly includes first and second endplates at opposite ends of the filter assembly. A first moveable subassembly includes at least a first x-ray filter and is configured to move along an axis perpendicular to the first endplate between the first the second endplates. A second moveable subassembly is also provided that includes at least a second x-ray filter. The second moveable subassembly is configured to move along an axis perpendicular to the second endplate between the first and second endplates. The first moveable subassembly and the second moveable subassembly are independently movable to provide at least a small bowtie x-ray filter, a large bowtie x-ray filter, a medium bowtie x-ray filter, a flat filter, and a closed position for a radiation source positioned in a fixed position relative to the filter assembly.
The bowtie filters are used to shape the intensity profile of an x-ray beam of the computed tomography apparatus. The intensity profile is shaped to compensate for the shape of a person's body, in particular, the intensity profile is shaped such that more photons are directed to a center of a person than to a periphery of a person, because radiation directed onto the center of the person is generally attenuated much more than the radiation directed onto the periphery of the person.
The computed tomography apparatus comprises an x-ray tube for emitting a conical x-ray beam which is filtered by the filter assembly. The filtered x-ray beam traverses a person from different directions and projection data are detected depending on the intensity of the filtered x-ray beam after having traversed the person. The computed tomography apparatus is adapted to reconstruct an image of the person from the detected projection data. The reconstructed images show artifacts which diminish the quality of the reconstructed images.